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References: <20240426121349.97651-1-puranjay@kernel.org> <20240426121349.97651-3-puranjay@kernel.org>
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From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
Date: Fri, 26 Apr 2024 09:26:42 -0700
Message-ID: <CAEf4BzaNM5H3Ad2=Syhhq1cbfuB5FrtuFTZHPTdQP3QME3naKA@mail.gmail.com>
Subject: Re: [PATCH bpf-next v3 2/2] bpf, arm64: inline bpf_get_smp_processor_id()
 helper
To: Puranjay Mohan <puranjay@kernel.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will@kernel.org>, 
	Alexei Starovoitov <ast@kernel.org>, Daniel Borkmann <daniel@iogearbox.net>, 
	Andrii Nakryiko <andrii@kernel.org>, Martin KaFai Lau <martin.lau@linux.dev>, 
	Eduard Zingerman <eddyz87@gmail.com>, Song Liu <song@kernel.org>, 
	Yonghong Song <yonghong.song@linux.dev>, John Fastabend <john.fastabend@gmail.com>, 
	KP Singh <kpsingh@kernel.org>, Stanislav Fomichev <sdf@google.com>, Hao Luo <haoluo@google.com>, 
	Jiri Olsa <jolsa@kernel.org>, Zi Shen Lim <zlim.lnx@gmail.com>, Xu Kuohai <xukuohai@huawei.com>, 
	Florent Revest <revest@chromium.org>, linux-arm-kernel@lists.infradead.org, 
	linux-kernel@vger.kernel.org, bpf@vger.kernel.org, puranjay12@gmail.com
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On Fri, Apr 26, 2024 at 5:14=E2=80=AFAM Puranjay Mohan <puranjay@kernel.org=
> wrote:
>
> As ARM64 JIT now implements BPF_MOV64_PERCPU_REG instruction, inline
> bpf_get_smp_processor_id().
>
> ARM64 uses the per-cpu variable cpu_number to store the cpu id.
>
> Here is how the BPF and ARM64 JITed assembly changes after this commit:
>
>                                          BPF
>                                         =3D=3D=3D=3D=3D
>               BEFORE                                       AFTER
>              --------                                     -------
>
> int cpu =3D bpf_get_smp_processor_id();           int cpu =3D bpf_get_smp=
_processor_id();
> (85) call bpf_get_smp_processor_id#229032       (18) r0 =3D 0xffff8000820=
72008
>                                                 (bf) r0 =3D &(void __perc=
pu *)(r0)
>                                                 (61) r0 =3D *(u32 *)(r0 +=
0)
>
>                                       ARM64 JIT
>                                      =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
>
>               BEFORE                                       AFTER
>              --------                                     -------
>
> int cpu =3D bpf_get_smp_processor_id();           int cpu =3D bpf_get_smp=
_processor_id();
> mov     x10, #0xfffffffffffff4d0                mov     x7, #0xffff8000ff=
ffffff
> movk    x10, #0x802b, lsl #16                   movk    x7, #0x8207, lsl =
#16
> movk    x10, #0x8000, lsl #32                   movk    x7, #0x2008
> blr     x10                                     mrs     x10, tpidr_el1
> add     x7, x0, #0x0                            add     x7, x7, x10
>                                                 ldr     w7, [x7]
>
> Performance improvement using benchmark[1]
>
>              BEFORE                                       AFTER
>             --------                                     -------
>
> glob-arr-inc   :   23.817 =C2=B1 0.019M/s      glob-arr-inc   :   24.631 =
=C2=B1 0.027M/s
> arr-inc        :   23.253 =C2=B1 0.019M/s      arr-inc        :   23.742 =
=C2=B1 0.023M/s
> hash-inc       :   12.258 =C2=B1 0.010M/s      hash-inc       :   12.625 =
=C2=B1 0.004M/s
>
> [1] https://github.com/anakryiko/linux/commit/8dec900975ef
>
> Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
> Acked-by: Andrii Nakryiko <andrii@kernel.org>
> ---
>  kernel/bpf/verifier.c | 24 +++++++++++++++++-------
>  1 file changed, 17 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 4e474ef44e9c..6ff4e63b2ef2 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -20273,20 +20273,31 @@ static int do_misc_fixups(struct bpf_verifier_e=
nv *env)
>                         goto next_insn;
>                 }
>
> -#ifdef CONFIG_X86_64
>                 /* Implement bpf_get_smp_processor_id() inline. */
>                 if (insn->imm =3D=3D BPF_FUNC_get_smp_processor_id &&
>                     prog->jit_requested && bpf_jit_supports_percpu_insn()=
) {
>                         /* BPF_FUNC_get_smp_processor_id inlining is an
> -                        * optimization, so if pcpu_hot.cpu_number is eve=
r
> +                        * optimization, so if cpu_number_addr is ever
>                          * changed in some incompatible and hard to suppo=
rt
>                          * way, it's fine to back out this inlining logic
>                          */
> -                       insn_buf[0] =3D BPF_MOV32_IMM(BPF_REG_0, (u32)(un=
signed long)&pcpu_hot.cpu_number);
> -                       insn_buf[1] =3D BPF_MOV64_PERCPU_REG(BPF_REG_0, B=
PF_REG_0);
> -                       insn_buf[2] =3D BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF=
_REG_0, 0);
> -                       cnt =3D 3;
> +                       u64 cpu_number_addr;
>
> +#if defined(CONFIG_X86_64)
> +                       cpu_number_addr =3D (u64)&pcpu_hot.cpu_number;
> +#elif defined(CONFIG_ARM64)
> +                       cpu_number_addr =3D (u64)&cpu_number;
> +#else
> +                       goto next_insn;
> +#endif
> +                       struct bpf_insn ld_cpu_number_addr[2] =3D {
> +                               BPF_LD_IMM64(BPF_REG_0, cpu_number_addr)
> +                       };

here we are violating C89 requirement to have a single block of
variable declarations by mixing variables and statements. I'm
surprised this is not triggering any build errors on !arm64 &&
!x86_64.

I think we can declare this BPF_LD_IMM64 instruction with zero "addr".
And then update

ld_cpu_number_addr[0].imm =3D (u32)cpu_number_addr;
ld_cpu_number_addr[1].imm =3D (u32)(cpu_number_addr >> 32);

WDYT?

nit: I'd rename ld_cpu_number_addr to ld_insn or something short like that

> +                       insn_buf[0] =3D ld_cpu_number_addr[0];
> +                       insn_buf[1] =3D ld_cpu_number_addr[1];
> +                       insn_buf[2] =3D BPF_MOV64_PERCPU_REG(BPF_REG_0, B=
PF_REG_0);
> +                       insn_buf[3] =3D BPF_LDX_MEM(BPF_W, BPF_REG_0, BPF=
_REG_0, 0);
> +                       cnt =3D 4;

nit: we normally have an empty line here to separate setting up
replacement instructions from actual patching

>                         new_prog =3D bpf_patch_insn_data(env, i + delta, =
insn_buf, cnt);
>                         if (!new_prog)
>                                 return -ENOMEM;
> @@ -20296,7 +20307,6 @@ static int do_misc_fixups(struct bpf_verifier_env=
 *env)
>                         insn      =3D new_prog->insnsi + i + delta;
>                         goto next_insn;
>                 }
> -#endif
>                 /* Implement bpf_get_func_arg inline. */
>                 if (prog_type =3D=3D BPF_PROG_TYPE_TRACING &&
>                     insn->imm =3D=3D BPF_FUNC_get_func_arg) {
> --
> 2.40.1
>